Elimination of enamel fishscaling in iron and steel sheets



United States Pate O b 3,265,541 ELIMINATION OF ENAMEL FISHSCALING INIRON AND STEEL SHEETS James A. Elias, Middletown, Ohio, assignor toArmco Steel Corporation, Middletown, Ohio, a corporation of Ohio NoDrawing. Filed Sept. 16, 1963, Ser. No. 309,342 9 (llaims. (Cl.Hit-16.6)

This invention has to do with treatments for ferrous metallic sheetswhich will result in a product with a slight degree of porosity. Thisapplication is a continuationin-part of the copending case in the nameof the same inventor, entitled, Elimination of Fishscaling in Iron andSteel Sheets, filed March 29, 1962, as Serial No. 183,405, nowabandoned.

A primary object of the invention is the provision of a simple andinexpensive treatment for ferrous sheets to be used in vitreousenameling, which sheets will be devoid of fishscaling. The invention isnot limited to this field of utility, as a material having a slightlyporous surface finds use in other fields,-such as providing an improved9 base for the reception of metallic coatings.- However, the followingdescription will be directed to theproduction of articleswhich are to becoated with a single. fired vitreous enamel.

Where a product is toibe made with a first fired coata ing which isfollowed by another separately fired coating, it is generally-possiblenot only to secure adequate adhesion by proper choice of-the compositionof the first fired coating, but also to produce'a more acceptableproduct because many of the defects which may be encountered in thefirst coating are either masked by the second coating or are alleviatedby fusion when the second coating is fired. I I

However, the present invention is useful in the field (2) The phenomenonofprimar y boiling must-be obviated, and Y t,

(3) The phenomenon offishscaling must also be prevented from occurring.I 5

The first of these problems gene-rally adequately products receivingmorethan one separately fired'c'oathandled by careful techniques, byroughening the surfaces of the stock, and by proper selection of theformulafor the frit which is to be fired to form the vitreous enamellayer.

The primary boiling of an enamel coating-has been traced to carbon atthe surfaces of the metal; and-it is lems l and 2 have been developed,and reference may be made to U.S. Patent No. 2,755,210, issued July 17,1956, in the names of Sutphen et al.

The third problem manifests itself in the tendency of a coating ofvitreous enamel to form fishscales. As the vitreous material cools, itbecomes friable and will tend to spall out and expose the base metal insmall halfmoon areas resembling fish scales. It is difiicult if notimpossible to predict the fishscaling tendencies of any given enamelingstock, and since any fishscaling will ruin the ultimate product, it isvital to take steps to eliminate any fishscaling tendencies the stockmay have.

white or light colored-enameled products,

Fishscaling is promoted in areas Where the adherence of theenamel hasbeen damaged by metal finishing operations such as grinding, or byscratches. Fishscaling generally occurs upon the initial cooling of theenamel coating, but in some instances its appearance may be delayed fora number of hoursor days.

Basically, however, the phenomenon of fishscaling appears to be due tosome characteristic of the metal base itself, and primarily to itssurface condition. Beall et al. in US. Patent No. 2,878,151, issued-March 17, 1959, found that fishscaling could bealleviated by a procedurewhich involved the formation of massive carbides in hot rolled stock,followed by coldreduction to elongate and fragment the massive carbides;presumably leaving the stock in a porous condition, after which thefragmented carbide inclusions were largely or completely removed bydecarburization. According to one theory, the pores or voids in thestock. provide a reservoir for hydrogen, which prevents fishscaling. Theelimination of fishscaling by this method involves extra steps in arouting-for enameling stock. r

It is a-primary-object of this inventionto provide a ferrousstock whichhas numerous small voids adjacent the surface.

-It is an object of thisinvention to provide a way of eliminating enamelfishscaling which does not necessarily add ext-ra'steps to a normalrouting.

It is .an object of thisinvention to provide-for the elimination ofenamel fishscaling in a new, simple and inexpensive way. r

It is an object of the invention to provide a treatment for theelimination of enamel fishscaling which does not nterfere with theconcurrent. or simultaneous practice fiaf other steps, for the solutionof others of the problems set forth above. 1 a

It is an object of the invention to provide a new and improved type ofenameling stock devoid of the fishscaling difficulty, and more narrowlyto provide a stock which is eminently suitable for the formation ofsingle-fired These and other objects of the invention which will be setforth hereinafter or will be apparent to one skilled inthe art uponreading these specifications, are accomplied by that procedure and inthat article of which exemplary embodiments will now be set forth. 7

It has been discoveredthat a ferrous enameling stock which will bedevoid of the fishscaling tendency can be attained in a singleheattreatment by a control of the atmosphere of the furnace, andinparticular by successive stages therein which are most convenientlycalled 'nitridingl and denitriding. Without intent to be bound bytheory, it isbelieved that the basic result of the process stepshereinafter outlined. is the provision of porosity adjacent the surfaceof the sheet stock, which porosity .is of such character .andextent asto be effective in the prevention of fishscaling, s v

There has hithertobeen a practice of nitriding steel alloy articles toimpart surface hardness to them. The

alloys of which such articles are made usually contain about 1%aluminum, with or without chomium, molybdenum and other strong nitrideformers; and articles .produced from such alloys have hitherto beensubjected to a heat treatment between about 950" F. and 1050 F.

for about 48 hours in an atmosphere of ammonia for thepurpose ofproducing on their surfaces a very hard case which. will resist wear,the case usually having a thickness ranging from between .010" and.020". Also use of an open or continuous heat treatment.

example, when it is desired to soften a case-hardened alloy article forfurther machining, as taught for example, in U.S. Patent No. 1,864,083.

However, prior art procedures such as those outlined above are noteffective for the purposes of this invention and have not been attemptedfor the uses of this invention. There has been no recognition in the artor literature of the basic phenomena upon which the present invention isbased.

In the practice of this invention, special alloys are not employed, butinstead, ordinary irons or mild steels such as those hitherto in use forenameling purposes. Since the elimination of fishscaling is not heredependent upon the formation, fragmentation and subsequent removal ofmassive carbides, it is not necessary that the starting material have arelatively high carbon content. However, a stock may be used having acarbon content of the order of that of the auto body sheet stock ofcommerce which is about .O3% to .08%, especially since decarburizationmay be practiced as later described.

The principles of the present invention are applicable to stocks whichare either hot rolled or cold rolled, or reduced to gauge by acombination of hot rolling and cold rolling. The metal may be refined inany suitable way and by the use of any suitable melting and refiningfurnace including the open hearth, the electric furnace, or vessels inwhich the molten metal is blown with oxygen. In carefully controlledopen hearth practice, a carbon content as low as about .025 is readilyattained. With the exception of enough manganese to prevent hotshortness, as known in the art, alloying ingredients are not used. Anyquantities of phosphorus, sulfur, silicon, nickel, molybdenum, copper orother alloy ing ingredients present or preferably within the range ofordinary impurities incident to the steel making process.

In view of the composition of the iron or steel in the practice of thisinvention, it will be evident that nitrides formed during the nitridingstep hereinafter outlined will be essentially nitrides of iron. It isnecessary that such nitrides be formed in sufficient quantity to effecta permanent distortion of the iron lattice at the surface of the sheetstock. Upon the subsequent decomposition of some or all of the nitridecontent and the removal of the evolved nitrogen, the surface of thestock will be left in the desired porous condition.

As distinguished from conventional nitriding treatments, the first stageof the heat treatment of this invention will be carried on Within atemperature range of about 1200 F. to about 1500 F. (preferably in arange of about 1300 F. to about 1400 F.), for a period of about one hourin an atmosphere consisting of or containing ammonia. While in a furnacein which the stock can be handled in a single layer in the presence ofstreaming gas, and where times may be extended as desired, a relativelysmall percentage of ammonia in the annealing atmosphere may be used,such conditions are obtainable primarily in the laboratory. It has beenascertained that with rates of flow such as can be obtained incommercial furnaces, the desired effect can be produced within minutesat the temperatures set forth by the use of an atmosphere consisting ofammonia. Where the atmosphere contained only about 50% ammonia, atreatment of 60 minutes was required at temperature. Where theatmosphere contained only 25% ammonia, a treatment at temperature for120 minutes appeared to be marginal where the final effect was theelimination of fishscaling. It should be understood that the quantity ofammonia required for a given amount of nitriding is to all intents andpurposes constant; but if any given heat treatment the flow rate of theatmosphere for a given quantity of base metal is increased, thepercentage of ammonia in the atmosphere may be reduced.

It will be noted that'the times involved preclude the Heat treatments intightly wound coils or stacked sheets are not fully reliable since thefurnace gases are likely not to have free access to the surfaces of thestock whether or not an annealing separator is used.

The best method presently available for the practice of the heattreatment is loose coil annealing. In coiling the stock, a strand iswound into the coil between convolutions. In some instances the strandwill then be removed, leaving the coil convolutions in a separatedcondition. In an alternative and preferred procedure, a metal, wire-likeseparating'strand may be wound into the coil and left between theconvolutions during the annealing operation. The strand will be soconfigured as not only to separate the convolutions of the coil, but toprovide adequate space for gas movement past the separating strand. Aloose coil formed in either of these Ways may be annealed in amufiie-type furnace, where the coil will normally be supported on endupon a foraminous means through which the furnace :gases can pass. Somemufiie furnaces are provided with a positive device for circulating thefurnace atmosphere and directing it through the coil between theconvolutions. Means are provided for the introduction and withdrawal ofthe atmosphere, and for changing the atmosphere within the mufiie duringthe operation thereof.

The treatment of the stock in the way set forth will result in theformation of a case which can be seen under a microscope. However,unlike the case formed in conventional nitriding operations, thethickness of the surface skin in the practice of this invention ispreferably very much less, namely, of the order of .001" to .002. Itwill be understood that satisfactory results can be attained withthicker cases, such for example as cases measuring .003" to .005", butthis is unnecessary in most instances.

As previously indicated, nitriding steels, as hitherto known in the art,contain alloying substances, and the hardened surface or case consistsof a tempered martensitic structure containing a fine dispersion ofalloy nitrides. The stock employed in the practice [of this invention isa low carbon rimmed steel which does not contain substantial proportionsof nitride-forming alloying elements; and the case formed in the firststep of the heat treatment of this invention consists primarily of ironnitrides. The case will contain substantially all of the nitrogen whichhas been absorbed during the first stage of the heat treatment since thetimes involved are not long enough to permit extensive diffusion. Thusthe thickness of the case can be taken as an index of the extent of thenitriding.

It is difficult otherwise to indicate the extent of the nitridingrequired. The elimination of fishscaling can, however, be accomplishedby treating the stock as above set forth and with the formation of acase varying from about .001" to .002" in thickness. The gauge of thestock itself has an unexplained effect upon the requirements for theelimination of fishscaling, heavier gauges being advantageously treatedfor the production of heavier cases. The case thickness preferablyvaries directly with the stock thickness. But within the range of thelimits of case thickness set forth, fishscaling may be dependablyprevented over the entire gamut of gauges normally used for ferroussheet stock intended for vitreous enameling, and extending at least fromabout IZ-gauge to about 26-gauge.

Where, during the first or nitriding step of the heat treatment, theatmosphere does not consist entirely of ammonia, it is preferable thatthe remainder of the atmosphere be hydrogen. Other non-oxidizing gasesmay be employed, such as cracked ammonia, argon, DX gas and similarmixtures; but it will generally be desirable to avoid a gas which has acarburizing effect at the temperatures involved.

Decarburization may be desired, and a low surface carbon content is ofvalue in preventing primary boiling. Decarburization may be done inknown ways at any point in the processing of the stock, but ispreferably accomplished during the first, second or both stages of theheat treatment described herein (at the temperatures and within thetimes set forth) by using a wet atmosphere, i.e. by so treating theammonia or mixture of ammonia and other gases that the atmosphere willhave a dew point of about 125- F. By decarburization in the sense ofthis application is meant a reduction of the carbon content at least atthe surfaces of the stock to such a value that primary boiling will notoccur upon the application of a single fired coat of vitreous enamel.This is not necessarily a matter of total carbon content of the stock,since rimmed steels may often be enameled successfully when the carboncontent of the actual surfaces is low, even though the stock as a wholemay analyze .05% total carbon or more.

Y The next stage of the heat treatment is denitriding and isaccomplished by heating the previously nitrided stock in a non-oxidizinggas which is devoid of ammonia or which is characterized by a partialpressure of nitrogen lower than the partial pressure of nitrogen in thesteel itself. Again, the gaseous atmosphere is preferably hydrogen,although other non-oxidizing gases may be employed. It will beunderstood that diatomic nitrogen does not nitride the stock and may becontained as a diluent in the denitriding atmosphere. Ammonia, however,is a strong nitriding gas; and must be kept out of the denitridingatmosphere to as great an extent as possible. The hydrogen contained inthe atmosphere will tend to combine with monatomic nitrogen from thestock to produce ammonia. Thus the movement of the denitridingatmosphere over the surfaces of the stock must be rapid enough to keepthe quantity of ammonia very low; and the statement above concerning thepartial pressure of nitrogen should be understood in this context.

Where the heat treatment is being conducted with the material in opencoil form in a muifie furnace, as above described, the ammonia bearingatmosphere present in the mufile during the first stage will be sweptout of it and replaced by the fresh gas which is substantially devoid ofammonia. The same temperature range given above for the nitriding stagecan be employed during denitriding. While high temperatures may be usedfor the treatments of this invention, the A critical temperature may beconsidered a maximum for the retention of optimum drawing properties.

To obtain the results of this invention, the denitriding should resultin the decomposition of the greater part of the nitrides of iron presentin the stock as the result of the first stage treatment. Some residue ofnitrogen or nitrides in the stock is tolerable; but it has beenascertained that substantial quantities of nitrogen or nitrides in thestock will produce primary boiling. In this respect nitrides appear toact similarly to carbon and carbides. In other words, the product ofthis invention will preferably be free of any substantial quantityeither of carbon or nitrogen after full treatment; and hence it will befree of the tendency toward primary boiling. But the treatment outlinedherein, unlike a bare decarburization, is also effective in eliminatingthe tendency toward fishscaling.

The length of time required for denitriding is found to be substantiallythe same as that for nitriding at the same temperatures. The degree ofnitriding will depend both upon the quantity of ammonia in theatmosphere and the time of treatment at temperature, i3. the timeduration of that phase of the heat treatment during which the ammoniacontent is maintained in the atmosphere. Similarly in the denitridingphase, the time duration of the treatment in a hydrogen bearing gasdevoid of ammonia will vary with the extent of the nitriding, and thetemperabure. In general, for a heat treatment of minimum time duration,the quantity of ammonia for nitriding within the range given may varyinversely to the temperature,

while the denitriding time will also vary inversely to the temperaturewithin the temperature range set forth.

It is likely that the degree of nitrogen pickup necessary will varysomewhat depending upon the fishscaling tendencies of the difierentstocks. It has been found that .12% nitrogen will be adequate to preventdifficulties in most common materials in the form of ZO-gauge sheetstock. The density of iron is 7.87 grams per cubic centimeter as againstdensities of 6.34 and 6.57 respectively for the iron nitrides Fe N andFe N. The density of a sample such as the ZO-gauge sample referred towas measured before and after treatment. Comparing the product beforenitriding with the product after nitriding (with a nitrogen pickup of.l2%) and denitriding indicated a reduction in density amounting to .019gram per cubic centimeter. This reinforces the theory that as a nitrideparticle forms in the iron, an expansion occurs producing a plasticdeformation of the iron lattice surrounding the particle. When theparticle is subsequently wholly or partially reduced to metallic ironduring the denitriding step, the deformed lattice does not contract, anda void is produced. The voids are believed to be predominantlyintergranular. They are frequently visible under the microscope, butthis is not always so.

The tendency toward fishscaling will be found to be dif ferent indifferent sheet stocks having the same or approximately the sameformulae. Consequently, the minimum change in density to eliminatefishscaling can be expected to vary. However, uniformly satisfactoryresults may 'be expected on enameling stocks treated within the limitsabove set forth, which is to say, nitrided in an atmosphere containingfrom about 50% to ammonia at temperatures from about 1200 F. to 1500 F.for times varying from about 60 minutes to about 15 minutes (the timevarying inversely to the percentage of ammonia in the atmosphere) andthen denitrided at the same temperatures in a non-ammonia-bearing gasfor substantially the same lengths of time, in open :coil form in amuffle with normal gas flows of, say, 1800 c.f./hr. Higher rates of gasflow will permit either a reduction in treatment time or a reduction inthe percentage of ammonia in the gas.

It will be seen that a complete denitriding may not in .all instances benecessary for the formation of the desired voids, depending upon theextent of the original nitriding. For example, a 20-gauge materialproduced in accordance with this invention, and which exhibited nofishscaling when coated with a vitreous enamel frit and fired for 5minutes at 1600 F. was, upon analysis, found to contain .066% nitrogen.However, iron nitrides, when present in sufiicient quantity, are likelyto cause primary boiling, as has been pointed out. Consequently, areasonably thorough denitriding is prefrred; and the time of thedenitriding stage may be prolonged to whatever extent is required toproduce a stock which is not only non-fishscaling, but free of primaryboiling also.

Adjustments may be made in time, temperature, atmosphere and rate ofcirculation as may be found desirable, and these are comprised withinthe intended practice of the invention. But while it would be possibleto effect a given degree of nitriding in an atmosphere containing muchless than the stated minimum of ammonia, this would take so long a timeas to diminish seriously the commercial practicability of the process.In general, the process as herein taught can be carried on in a loosecoil muflle with a total holding time at temperature of about two hours.Both the nitriding and the denitriding steps together can beaccomplished Within the time required for an adequate decarburizationwhich might be six to ten hours in commercial practice. Where otherannealing equipment, or a separate furnace for each stage, is used, thetimes can be varied, or the two stages can be separated in point of timewith intervening cooling of the materialt The invention finds itsgreatest utility in the treatment of sheet stock made from rimmed ironor mild steel of the composition set forth above. The ferrous metal iscast into ingots which may be bloomed into sheet bars and hot rolled togauge in ways known in the art, or hot rolled to an intermediate gaugewith or without cooling and reheating of the metal at slab thicknesses.It is possible to apply the invention to killed steels, but thenitriding treatment may have to be longer or more intensive in order tonitride the metallic aluminum in the steel before the formation of thedescribed case.

Several exemplary routings are listed below for the production ofenameling stock.

- EXEMPLARY ROUTINGS (I) Hot roll to final gauge (3) Hot roll NormalizePickle Pickle Cold roll Nitride Nitride Denitride Denitride (2) Hot rollto final gauge (4) Hot roll Pickle Pickle Nitride Cold roll DenitrideNormalize Pickle Nitride Denitride Decarburization may be practiced, ifdesired, at any suitable point in the routing, including the nitridingand denitriding stages of the heat treatment.

The routings set forth above are relatively simple routings and, as willbe evident, are intended for a procedure in which the material is eitherhot rolled or cold rolled to a final gauge. The routings may be modifiedin various ways.

Irrespective of the manner in which the material is carried down tofinal gauge, various treatments for special purposes may be practiced.For example, some enameling stocks must be normalized, and a normalizingtreatment may be included in the routing. As set forthjn the Sutphen eta1. patent noted above, the surfaces of the stock subsequent to thenoted heat treatment may be coated with nickel, then heat treated in anoxidizing atmosphere to produce a scale, and then either pickled orannealed in a reducing atmosphere. In any instance Where it is founddesirable, a temper rolling involving a slight reduction may be added atthe end of the routing without destroying the porosity. The effect ofthe porosity is not destroyed by the forming or drawing procedures towhich enameling stocks are ordinarily subjected.

Example The material used in this test was ordinary, low-carbon rimmedsteel of 20-gauge. The stock had been reduced to gauge by theconventional combination of hot rolling and cold rolling, and it wasannealed in a 50 pound charge in a simulated open coil furnace with atotal gas flow of 2.7 c.f./hr. The annealing was done at 1300 F. in agas consisting of 40% hydrogen, 60% nitrogen, and a water contentsufficient to give a dew point of 120 F. The materialannealed in thismanner is representative of a full scale commercial anneal, and wasfound to be decarburized. However, when the material was coated withvitreous enamel in a single firing, severe fishscaling occurred.

Another portion of the same starting material was annealed in the sameapparatus in an atmosphere consisting of 50% ammonia, 20% hydrogen and30% nitrogen for one hour. After the first hour the ammonia waseliminated from the atmosphere, and the anneal was continued until thefinal carbon content reached 001%. Final portions of the anneal werecarried out in an atmosphere consisting of 40% hydrogen, 60% nitrogenwith a water content suflicient to give :a dew point of 120 F. The totalannealing time was about 4 hours in this test. It had been expectedafter the denitriding treatment that the nitrogen level would have beenlowered to the value of normal residual nitrogen, which is about 003%.In the particular test, however, the denitriding was not as efiicient ashad "been anticipated because water seals used with the furnace hadbecome contaminated with ammonia. The denitrided stock was found tocontain a total of .012% nitrogen. Nevertheless the stock enameled welland did not fishscale or show primary boiling.

This example is given as demonstrating the efficacy of nitriding anddenitriding for the elimination of fishscaling despite a somewhat higherresidual nitrogen value than is usual in furnace treatments of shorterduration wherein better control of the elimination of ammonia in thedenitriding atmosphere results both in a lower residual nitrogen and ina shortening of the time cycle to a value of about two hours, or less.

Modifications may be made in the invention without departing from thespirit of it. The invention having been described in various exemplaryembodiments, what is claimed as new and desired to be secured by LettersPatent is:

1. In a process of producing low'carbon iron or mild steel enamelingstock, the steps which consist in nitriding the stock in an ammoniacontaining annealing atmosphere to the extent of producing on thesurfaces thereof a case consisting essentially of iron and iron nitridesand having a thickness not less than substantially .001", and thendenitriding the stock in a hydrogen containing annealing atmospheresubstantially devoid of ammonia, including the step of effectingcontinuous flow of the denitriding atmosphere over the surfaces of thestock so as to substantially remove ammonia produced by nitrogen fromthe stock combining with hydrogen in the annealing atmosphere, wherebyto produce a stock characterized by porosity, which stock issubstantially devoid of vitreous enamel fishscaling tendency.

2. In a process of producing low carbon iron or mild steel enamelingstock, the steps which consist in first heat treating the final gaugestock to a temperature of substantially 1200 F. to substantially 1500F., first in an annealing atmosphere non-oxidizing to iron, andcontaining from substantially 50% to substantially ammonia whereby tonitride the stock, and second in a hydrogen bearing annealing atmospheresubstantially devoid of rammonia, including the step of maintaining thepartial pressure of nitrogen in the last named atmosphere normally lowerthan the partial pressure of nitrogen in the stock, whereby to denitridethe stock, the final product being a low carbon ferrous material devoidof primary boiling and fishscaling tendencies when coated with vitreousenamel.

3. The process claimed in claim 2 in which at least one of saidatmospheres contains nitrogen in diatomic form.

4. The process claimed in claim 3 wherein the times of treatment attemperature in each of the first and second steps is about one hour.

5. The process claimed in claim 3 wherein the times of treatment attemperature in the first and second steps vary from substantially 15minutes to substantially one hour, the quantity of ammonia in the firststep varying inversely with the time at temperature.

6. The process of making ferrous sheet stock for enameling whichcomprises reducing a low carbon mild steel to finished sheet gauge, andsubjecting the said stock in loose coil form to a heat treatment at atemperature of substantially 1300 F. to 1400 F. in an annealingatmosphere which during a first phase of the heat treatment is anatmosphere non-oxidizing to iron, containing from substantially 50% tosubstantially 100% ammonia and having a dew point of substantially i25and in which a second phase of the heat treatment is a hydrogen bearingatmosphere substantially devoid of ammonia in which the partial pressureof nitrogen in the last named atmosphere is normally maintained lowerthan the partial pressure of nitrogen in the stock.

7. The process claimed in claim 6 wherein the said heat treatment iscarried on in a muflie furnace in which the atmosphere is changedintermediate the first and second phases.

8. The process claimed in claim 7 wherein the time duration of each ofthe first and second heat treatments is substantially one hour.

9. The process claimed in claim 7 wherein the time duration of the firstheat treatment is from substantially 15 minutes to substantially onehour, and in which the time duration of the second phase is at least 15minutes and long enough to reduce the nitrogen content of the stock toan extent to eliminate primary boiling resulting from residual nitrogenin the stock.

References Cited by the Examiner UNITED STATES PATENTS Merten 148-16.6Kautz 14812.1 X Eckel et a1 11753 Pfeiffer 148-16] Liedhoh'n 14816.6

Fast 148- 16 Beall et a1 148-12 Chu 148139 HYLAND BIZOT,-PrinmryExaminer.

DAVID L. RECK, Examiner.

15 C. N. LOVELL, Assistant Examiner.

1. IN A PROCESS OF PRODUCING LOW CARBON IRON OR MILD STEEL ENAMELINGSTOCK, THE STEPS WHICH CONSIST IN NITRIDING THE STOCK IN AN AMMONIACONTAINING ANNEALING ATMOSPHERE TO THE EXTENT OF PRODUCING ON THESURFACES THEREOF A CASE CONSISTING ESSENTIALLY OF IRON NITRIDES ANDHAVING A THICKNESS NOT LESS THAN SUBSTANTIALLY .001", AND THENDENITRIDING THE STOCK IN A HYDROGEN CONTAINING ANNEALING ATMOSPHERESUBSTANTIALLY DEVOID OF AMMONIA, INCLUDING THE STEP OF EFFECTINGCONTINUOUS FLOW OF THE DENITRIDING ATMOSPHERE OVER THE SURFACES OF THESTOCK SO AS TO SUBSTANTIALLY REMOVE AMMONIA PRODUCED BY NITROGEN FROMTHE STOCK COMBINING WITH HYDROGEN IN THE ANNEALING ATMOSPHERE, WHEREBYTO PRODUCE A STOCK CHARACTERIZED BY POROSITY, WHICH STOCK ISSUBSTANTIALLY DEVOID OF VITREOUS ENAMEL FISHSCALING TENDENCY.